This invention relates generally to heat shrinkable, relatively gas impermeable thermoplastic multilayer films which can be used in the form of a hermetically sealed bag for cook-in packaging meat products wherein the packaged meat is submerged in heated water for cooking and bonds or adheres with the film's inner surface. Under these conditions the bag film is shrinkable against the adhering meat and retains its integrity.
As used herein, "cook-in" refers to packaging material structurally capable of withstanding exposure to long and slow cooking conditions while containing a food product, for example submersion in water at 70.degree.-80.degree. C. (158.degree. C.-176.degree. F.) for 4-6 hours. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods which may be directly transferred to the consumer in this form. These types of foods may be consumed with or without warming. Cook-in packaging materials maintain seal integrity and in the case of multilayer films, are delamination resistant. Cook-in films must also be heat shrinkable under cook-in conditions so as to form a tightly fitting package.
Cook-in films preferably have a tendency for adhesion to the food product, thereby preventing "cook-out", which is the collection of juices between the outer surface of the food product and the inner surface of the film.
There are numerous other requirements for a meat-adherable cook-in shrink film including: delamination resistance, low oxygen permeability, heat shrinkability representing about 20-50% biaxial shrinkage at about 90.degree. C. (194.degree. F.), and for certain applications optical clarity.
In U.S. Pat. No. 4,606,922 the prior art has proposed for meat adhesion, a multilayer film inner surface of an ionomer of a metal salt neutralized copolymer of an olefin and a carboxylic acid, said ionomer having been treated with ionizing radiation to a dosage sufficient to cause bonding with the meat product. As described in Patent '922, the olefin is preferably ethylene and the carboxylic acid is preferably acrylic acid or methacrylic acid. A representative ionomeric material of this class is commercially available as Surlyn.TM. from the Du Pont Company, and described in detail in U.S. Pat. No. 3,355,319.
The aforementioned U.S. Pat. No. 4,606,922 also describes an irradiated multilayer food casing having an outer nylon layer over one or more interior layers and having a Surlyn food contact inner surface. For example, the structure may comprise nylon (outer)/adhesive/Surlyn (inner). Nylon is described as providing an oxygen barrier to impede the inward diversions of oxygen and to impart relatively high stuffing strength to the casing. However, this film is not shrinkable and must be stuffed with a flowable meat product to insure sufficient film contact. Also, the nylon outer layer loses some of its effectiveness as an oxygen barrier because of moisture uptake from the atmosphere.
U.S. Pat. No. 4,606,922 also describes a four layer shrinkable film comprising EVA/PVDC/EVA/ionomer. However, the necessary irradiation has a degrading effect on the vinyl chloride-vinylidene chloride copolymer.
Another commercially employed cook-in film is the type described in Oberle et al U.S. Pat. No. 4,469,742, comprising six layers irradiated to dosage of preferably at least 6 MR. The central barrier layer is hydrolyzed ethylene-vinyl acetate copolymer (EVOH), chosen because of its higher softening point and superior oxygen barrier properties as compared to the PVDC-type barrier layer commonly used in multilayer films for ambient temperature applications. On each side of the barrier layer is an adhesive layer such as chemically modified polyethylene, eg. Plexar. On the outer side of each adhesive layer is a shrink layer such as ethylene-vinyl acetate copolymer (EVA), and the outside (abuse) layer is also the EVA type having a vinyl acetate content of about 5-12 wt. %. The innermost (heat sealing) layer may for example be an ionomer. Processing of a six layer film requires a complex die and limited resin selection.
It will be apparent from the foregoing that these prior art cook-in films are complex both in terms of multiple materials and sophisticated manufacturing techniques, and some are not heat shrinkable.
A possible solution to the foregoing problems is a four layer film with EVOH as the barrier layer and Surlyn as the inner layer, but tests have shown that such prior art films with even only three layers, i.e., no meat adhesion layer, either would not biorient, would tend to delaminate during cooking or have poor optical properties or have unacceptably low shrink value for cook-in applications, or a combination of these problems. Certain of these problems may be overcome by the use of EVOH-polyamide blend core layers, but multilayer films of this type with EVA inner and outer layers have typically been found to delaminate under cook-in conditions.
A possible approach to this delamination problem is the use of adhesives, but adhesives suitable for EVOH would not be expected to biorient, or in heavier gauges might be expected to be hazy or they might not provide the required shrinkage.
One object of this invention is to provide an improved meat-adherable cook-in type multilayer plastic film which has less than six layers.
Another object of this invention is to provide a less-than-six layer meat-adherable cook-in plastic film which is relatively easy to process, i.e., may be readily bioriented.
Still another object of this invention is to provide a less-than-six layer meat adherable cook-in type film having high shrink, good optical properties, good inner layer adhesion, and no blocking problem.
A further object of this invention is to provide such a less-than-six layer meat adherable cook-in type film which does not require additives such as processing aids and does not require high irradiation dosage levels on the order of 6 MR to realize high strength.
Other objects and advantages will be apparent from the ensuing disclosure and appended claims.